The invention relates to methods and apparatus for accessing the ability of a vehicle emissions control device, such as a lean NOx trap, to releasably store an exhaust gas constituent and, more particularly, to a method and apparatus for estimating the capacity of a lean NOx trap to store NOx.
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
To limit the amount of feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream three-way catalyst and a downstream NOx absorbent or xe2x80x9ctrap.xe2x80x9d The three-way catalyst is particularly effective at reducing tailpipe NOx emissions when the engine is operated using an air-fuel mixture that is at or near a stoichiometric air-fuel ratio. The trap, in turn, stores NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and releases previously-stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry. In this manner, the trap permits intermittent lean engine operation, with a view toward maximizing overall fuel economy, while concomitantly serving to control vehicle tailpipe emissions.
More specifically, in a typical embodiment, the trap chemically stores NOx during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat. The NOx (NO and NO2) are stored in the trap in the form of barium nitrate, for example. The washcoat also includes precious metals, such as platinum and palladium, which operate to convert NO to NO2 for storage in the trap as a nitrate. The trap""s washcoat typically also includes ceria, whose affinity for oxygen storage is such that, during initial lean engine operation, a quantity of the excess oxygen flowing through the trap is immediately stored in the trap. The amount of stored oxygen is essentially fixed, although it begins to lessen over time due to such factors as increased trap sulfurization (sulfur accumulation) and trap aging.
The trap""s actual capacity to store NOx is finite and, hence, in order to maintain low tailpipe NOx emissions when running xe2x80x9clean,xe2x80x9d the trap must be periodically cleansed or xe2x80x9cpurgedxe2x80x9d of stored NOx. During the purge event, excess feedgas HC and CO, which are initially consumed in the three-way catalyst to release stored oxygen, ultimately xe2x80x9cbreak throughxe2x80x9d the three-way catalyst and enter the trap, whereupon the trap""s barium nitrate decomposes into NO2 for subsequent conversion by the trap""s precious metals into harmless N2 and O2. The oxygen previously stored in the trap is also released during an initial portion of the purge event after the HC and CO break through the three-way catalyst.
Each purge event is characterized by a fuel xe2x80x9cpenaltyxe2x80x9d consisting generally of an amount of fuel required to release both the oxygen stored in the three-way catalyst, and the oxygen and NOx stored in the trap. Moreover, the trap""s NOx-storage capacity is known to decline in a generally reversible manner over time due to sulfur poisoning or xe2x80x9csulfurization,xe2x80x9d and in a generally irreversible manner over time due, for example, to component xe2x80x9cagingxe2x80x9d from thermal effects and xe2x80x9cdeep-diffusionxe2x80x9d/xe2x80x9cpermanentxe2x80x9d sulfurization. As the trap""s capacity drops, the trap is xe2x80x9cfilledxe2x80x9d more quickly, and trap purge events are scheduled with ever-increasing frequency. This, in turn, increases the overall fuel penalty associated with lean engine operation, thereby further reducing the overall fuel economy benefit of xe2x80x9crunning lean.xe2x80x9d
In order to restore trap capacity, a trap desulfurization event is ultimately scheduled, during which additional fuel is used to heat the trap to a relatively elevated temperature, whereupon a slightly rich air-fuel mixture is provided for a relatively extended period of time to release much of the stored sulfur and rejuvenate the trap. As with each purge event, each desulfurization event typically includes the further xe2x80x9cfuel penaltyxe2x80x9d associated with the initial release of oxygen previously stored in the three-way catalyst and the trap. The prior art teaches scheduling a desulfurization event only when the trap""s NOx-storage capacity falls below a critical level, thereby minimizing the frequency at which such further fuel economy xe2x80x9cpenaltiesxe2x80x9d are incurred.
Accordingly, there is a need for a method and apparatus for accurately determining the NOx-storage capacity or efficiency of a lean NOx trap in order to accurately schedule the desulfurization event as well as the purge event.
In accordance with the method of the present invention, the NOx absorption capacity is determined based on an estimate of the change in the oxygen storage capacity of the lean NOx trap. More particularly, after a desulfurization event is performed, to put the lean NOx trap in a known state, a number of estimates of the current value of oxygen storage capacity are determined in order to calculate a filtered or mean value of the oxygen storage capacity of the lean NOx trap when it is fresh. This initial capacity value is then stored in computer memory as a value P1 and also as a value P2 representing the current oxygen storage capacity of the trap. Subsequently, and at periodic time intervals the value of the current oxygen capacity of the trap is estimated and filtered and the value P2 is updated. The current trap capacity to absorb NOx is then determined as a function of the value of P2/P1. When the trap capacity to absorb NOx falls below a predetermined minimum capacity value, a desulfurization event is performed and the forgoing steps are repeated.